Experimental And Theoretical Studies Of The Catalytic CO Oxidation By Gold Supported On The Semi-polar ZnO{101} Surfaces

Catalytic oxidation of carbon monoxide（CO）is a prototypical reaction in heterogeneous catalysis.Gold nanoparticles supported by oxides have exhibited remarkably catalytic performance for CO oxidation at low or room temperature.It is generally accepted that the interface between gold catalyst and oxide support plays a vital role in this process,which is greatly influenced by the strong metal-support interaction（SMSI）.On the one hand,SMSI favors the high dispersion of gold catalyst and their catalytic activity as well.On the other hand,encapsulation effect of metal catalyst by oxide by SMSI results in the decrease of the catalytic activity.It is expected to improve the catalytic activity of CO oxidation by limiting the interfacial encapsulation of gold catalysts epitaxially achored on special surface of the oxide.In this thesis,taking gold supported by zinc oxide（Au/ZnO）as an example,we perform experimental and theoretical studies of the catalytic CO oxidation by gold nanoparticles/nanoclusters supported on the semi-polar ZnO{101}surfaces.Firstly,the hexagonal pyramidal ZnO support is synthesized by solvothermal method,exposing mainly the{101}surfaces,accounting for 65%.The Au/ZnO catalyst is synthesized via a deposition-precipitation method,characterized by scanning electron microscope（SEM）and transmission electron microscopy（TEM）.Results show that,small gold nanoparticles are selectively dispersed on the ZnO{101}surfaces.The Au/ZnO catalyst is pretreated in high temperature（300°C）O2 and H2atmospheres referring to oxidative SMSI（O-SMSI）and reductive SMSI（R-SMSI）,respectively.The CO oxidation reaction is tested in a fix-bed flow reactor system.Results indicate that the catalytic CO conversion with O-SMSI is obviously higher than that with R-SMSI.Secondly,density functional theory（DFT）calculations are performed to comparatively investigate the O-SMSI and R-SMSI using a small gold nanocluster（Au10）supported on the stable O-rich and Zn-rich ZnO（101）surfaces,respectively,In addition,the O-terminated ZnO（101）is also used as a reference modeling the neutral SMSI（N-SMSI）.Results show that catalytic reactivity of CO oxidation follows the order of O-SMSI>N-SMSI>R-SMSI.The rate-determining activation barriers are 0.93,1.16,and 1.25 eV,respectively,which are linearly correlated with the binding energies（1.26,0.96,and 0.82 eV）of Au10 on the ZnO{101}surfaces.The stronger the SMSI,the higher the activity.Besides,O-SMSI,N-SMSI and R-SMSI have great influences on the band energy level of Au-5d of Au₁₀ cluster.The d-band center ofε（Au-5d）follows the order of-2.45 eV（O-SMSI）>-2.52 eV（N-SMSI）>-2.65 eV（R-SMSI）,in good agreement with the surface work function of the ZnO{101}surface,5.27 eV（O-SMSI）>2.63 eV（N-SMSI）>1.68 eV（R-SMSI）.Moreover,the energies of the transition states are linearly related with theε（Au-5d）value,indicating that O-SMSI favors to increase the reactivity the Au-5d electrons and the catalytic activity of gold for CO oxidation.Finally,based on the principle of ab initio thermodynamics,the most stable O-and Zn-terminated ZnO{101}surfaces are used for constructing a generalized chemical potential model of small gold nanoclusters(Au₅ and Au₁₀)to evaluate their thermodynamic stabilities.Results show that the Au₅ and Au₁₀ clusters are more stable on the O-terminated ZnO（101）surface than those on the Zn-terminated counterpart.Moreover,the mechanism of CO oxidation is investigated using the stable gold clusters on the O-terminated ZnO（101）considering the influence of H/OH species around the Au/ZnO interface.Results indicate that with the help of the H/OH,both Au₅ and Au₁₀ present better activities for the dissociation of O₂,which thus improves the catalytic activity for CO oxidation.